An ordinary dc series motor ,when connected to single-phase ac supply,operates,bu not very satisfactorily.The reasons for this are as follows:
- The torque developed is not of constant magnitude,as in dc series motors,but pulsates between zero and maximum values each half cycle because the field and rotor currents both reverse every half cycle.
- The alternating filed flux would induce eddy currents in the solid parts of the field structure,such as yoke and cores,causing excessive heating and lowering of efficiency.
- There is no inductive coupling between the field(stator) and armature(rotor) winding,as they are placed in space quadrature.As a result,both the windings act as open circuited primaries of the Transformer and therefore each winding has large reactance.This results in small current and abnormal voltage drop and so reduces the output and power factor to such low values as to make the motor impractical.
- When a series motor operates on dc supply,it develops rotational emf in conductors placed along the brush or q-axis(along AB) due to the armature reaction flux.Reactance voltage i.e self induced emf is developed in these conductors due to the reversal of the current in them on crossing the brush.But in case of ac operation,in addition to rotational emf and reactance voltage, transformer emf is also developed which will be maximum for conductors A and B because armature coil with the greatest emf is the one whose magnetic axis is parallel to the main field.The voltage developed by the transformer action are greatest between segments at commutator where the brushes are situated called a neutral points.This would cause a destructive sparking.
- Because of the larger voltage drop in the machine due to its high inductance the speed for ac operation is much less than that for dc operation for a given torque.
- Starting torque is also poor due to low power factor at starting.
Modification and Improvement in design of AC series motor
There are several modification necessary in dc series motor to operate satisfactorily on ac.
- In order to reduce the eddy current losses,the whole magnetic circuit is laminated,the field magnet system as well as the armature core.
- In order to reduce the reactance of the series field,ac series motors are built with a few turns as possible.Fewer filed turns per pole means lower flux per pole which improves the commutation by reducing the transformer emf in the commutation coils.
- Reduction in number of turns on the field winding would also reduce the flux for a given current and would tend to increase the speed and reduce the torque available for a given current.Hence to develop the required load torque the number of armature conductors would have to be increased proportionately.
- The operating voltage is kept low in order to reduce the inductance.
- Since inductive reactance is directly proportional to frequency ,ac series motor operating characteristics are better at low frequency.
- Interpoles are provided in ac series motors to eliminate the armature-resistance leads,which occupy valuable space and lower the rating of the motor by their heating.
Reduction in number of turns on the field winding would also reduce the flux for a given current and would tend to increase the speed and reduce the torque available for a given current.Hence to overcome this problem there is the use of compensating winding.i.e
- Conductively compensated series motor
- Inductively compensated series motor
- Conductively compensated series motor:Excessive sparking and poor power factor are prevented by providing compensating winding.
The field flux is produced along the axis CD and armature flux is produced along axis AB.Thus the compensated winding must produces a flux along AB and in a direction opposite to that of the armature flux at every instant.The compensating winding is placed in the stator slots,and its axis is kept at 90 electrical degrees to the main filed axis.The number of turns per pole in the compensating winding is so choosen that the mmf produced by it is equal and opposite to that of the armature.The compensating winding carries current equal to that flowing in the armature but in opposite direction due to conductive effect called conductively compensated series motor.
2.Inductively compensated series motor:Another method is to short circuit the compensated winding,which behaves like short-circuited secondary of a transformer of which the primary is the armature winding.So when the compensated winding is excited inductively and the motor with short-circuited compensating winding is known as inductively compensated series motor.The current in the compensating winding will be almost directly proportional to the armature current and practically 180 degree out of phase with it.
All the ac series motor are provided with commutating poles for improving commutation but commutating poles alone don’t produce satisfactory commutation.
The speed emf and the transformer emf in the commutated coils add in quadrature to give a resultant emf.By shunting the interpoles with a non-inductive shunt resistor,the phase of the interpole flux can be made such as to neutralize the resultant.